1. Field of the Invention
The present invention relates to a rotary head drum unit having stationary upper and lower drums, to be applied to a recording/reproducing apparatus, such as a VTR.
2. Description of the Prior Art
The applicant of the present patent application has previously proposed a rotary head drum unit for a VTR in Japanese Utility Model Laid-open (Kokai) No. Sho 62-73337. As shown in FIGS. 7 to 10C, in this previously proposed rotary head drum unit, a plurality of rotary heads 4, i.e., magnetic heads, are inserted between an upper drum 2 and a lower drum 3 fixedly connected to each other by a drum post 1 (FIG. 8). A rotary flange, not shown, for rotatively driving the rotary heads 4 is disposed coaxially with the upper drum 2 and the lower drum 3. The diameter of the upper drum 2 is greater than that of the lower drum 3. A taper surface 6 is formed in the lower portion of the upper drum 2, and a reduced portion 5 of a diameter equal to that of the lower drum 3 is formed contiguously with the lower end of the taper surface 6. The taper surface 6 and the reduced portion 5 are symmetrical with respect to the axis of the upper drum 2. A helical tape lead or guide surface 7 is formed in the circumference of the lower drum 3. A magnetic tape 8 is guided by a lead-out guide 9 and a lead-in guide 10 (FIG. 8) so as to extend helically across the upper drum 2 and the lower drum 3 with the lower edge 8a thereof guided by the tape lead or guide surface 7. The arc .theta..sub.1 of contact between the rotary head drum unit and the magnetic tape 8 is, for example, about 188.degree. and the data recording angle .theta..sub.2 of the rotary heads 4 is, for example, about 178.degree. .
In recording data on or reproducing recorded data from the magnetic tape 8 by rotating the rotary heads 4 in the direction of the arrow a at a high rotating speed while the magnetic tape 8 runs in the direction of the arrow b, the magnetic tape 8 is biased downward by the taper surface 6 of the upper drum 2 so that the lower edge 8a of the magnetic tape 8 slides along the tape lead or guide surface 7 as the magnetic tape 8 runs.
As is obvious from FIGS. 10A, 10B and 10C showing longitudinal sections of the rotary head drum unit at different angular positions, respectively, since a portion of the upper drum 2 having the taper surface 6 extends in a plane perpendicular to the axis of the upper drum 2, the vertical position of the magnetic tape 8 moves downward relative to the taper surface 6 as the magnetic tape advances. Since the portion of the upper drum 2 having the taper surface 6 extends in a plane perpendicular to the axis of the upper drum 2 whereas the magnetic tape 8 runs obliquely to the taper surface 6, the upper edge 8b of the magnetic tape 8 deviates completely downward from the taper surface 6 in a lead-out region 11 near a departing position where the magnetic tape 8 departs from the rotary head drum unit. Accordingly, the taper surface 6 is unable to urge the magnetic tape 8 downward in the lead-out region 11 and, consequently, the lower edge 8a of the magnetic tape 8 is liable to separate upward from the tape lead or guide surface 7 and the magnetic tape 8 is liable to run unstably. In the lead-out region 11 near the departing position, a portion of the magnetic tape 8 near the upper edge 8b is liable to slack (FIG. 9) and, consequently, the magnetic tape 8 is liable to be in faulty contact with the rotary head 4.